The present invention relates to a process for detecting mechanical defects in a semiconductor device, such as a solar cell or solar cell arrangement.
In the case of solar generators for satellites, it is extremely important to detect mechanical defects in the solar cells before they are deployed in orbit. The reason is that mechanical or cell defects, such as ruptures, may continue to grow, principally as a result of the thermal cycles in orbit (90 eclipse phases per year in one geostationary orbit), and have an extremely negative effect on the reliability of the solar generators. It is therefore desired to achieve a 100% control of the cells with respect to mechanical defects before shipment (including all production steps), in order to exchange defective cells as early as possible and to avoid defective cells' being used in a solar generator. The same considerations of course also apply to terrestrial solar cells and modules even though, in contrast to the solar cells of a satellite in orbit, they can be exchanged more easily.
Currently, mechanical cell defects in solar cells made of indirect semiconductors, such as silicon, are detected by a visual cell inspection. That is, by looking at the solar cells under suitable illumination conditions, a trained eye can detect cell defects in reflected light, due to light effects caused by a slight vertical offset along the cell rupture. Since visual cell inspection is time-consuming and correspondingly expensive, however, efforts are being made to automate this process, for example, using phase-measuring deflectrometry. It has also been attempted to use external light of differing wavelengths in order to achieve a greater penetration depth, or to detect ruptures based on their effect on the transfer of heat, for example using thermographic methods. A survey of the detection of mechanical defects in solar cells can be found in the publication “Utilizing Lateral Current Spreading in Multijunction Solar Cells: An Alternative Approach to Detecting Mechanical Defects”, C. G. Zimmermann, J. Appl. Phys. 100, 023714, 2006.
European Patent Document EP 1 416 288 B1 describes a process for optical detection of mechanical defects in semiconductor devices (particularly solar cell arrangements) which have at least one pn junction and at least one semiconductor layer made of a semiconductor material with a direct band junction. Operating the pn junction in the transmitting direction, affects the active radiation behavior, which can then be analyzed for the detection of mechanical defects. In contrast to the visual inspection process described above, the semiconductor device is thus not passively irradiated; rather, radiation behavior of the device itself is actively excited. The radiation behavior is affected by the radiating recombination of charge carriers at the direct band junction which are induced in the pn junction polarized in the transmitting direction. According to European Patent Document EP 1 416 288 B1 such detection is not possible in the case of semiconductor material such as germanium, with an indirect band junction, due to insufficient radiating recombination of charge carriers.
One object of the present invention therefore, is to provide an improved method and apparatus for detection of defects in semiconductor devices with an indirect semiconductor material, for example, silicon-based solar cells or solar cell arrangements.
This and other objects and advantages are achieved by the method and apparatus according to the present invention, which is based on the recognition that even indirect semiconductors such as silicon have an analyzable radiation behavior under certain conditions. The publication “Photographic Surveying of Minority Carrier Diffusion Length in Polycrystalline Silicon Solar Cells by Electroluminescence”, T. Fuyuki, H. Kondo, T. Yamazaki, Y. Takahashi and Y. Uraoka, Appl. Phys. Lett. 86, 262108, 2005, indicates that the minority carrier diffusion length in polycrystalline silicon can be determined based on electroluminescence, and that therefore even indirect semiconductors supply sufficient analyzable electroluminescence. According to the invention, this recognition is now utilized in order to be able to use the process known from European Patent Document EP 1 416 288 B1, with a corresponding modification, also for the detection of mechanical defects in semiconductor devices with indirect semiconductor material.
By means of the invention, mechanical defects in a semiconductor device with a pn junction, which is based on an indirect semiconductor material (such as silicon), can therefore be detected by applying a voltage to this semiconductor device in the transmitting direction of the pn junction, so that the semiconductor device is excited to supply electromagnetic radiation. As a result of the indirect band junction, the resulting electroluminescence, however, is weaker by orders of magnitude than in direct semiconductors.
Optical detection of radiation emitted by the active layers in the semiconductor requires a suitable optical sensor, such as a CCD camera, which is adapted to the wavelength range of the determined radiation and, when integrated for an appropriate observation time, has a suitable sensitivity. For semiconductors based on Si or Ge (which are of considerable practical relevance), the emitted radiation will be in the infrared wavelength range. In this case, all visible light can be faded out by an infrared filter, so that relatively less sensitive detectors can also be used, and the detection of defects becomes possible in ambient light. In a more general case, suitable (band pass) filters can be used to isolate the radiation of a certain pn junction and to check only these semiconductor layers for defects. Optical detection in the sense of the present invention is particularly the local resolution of detected electromagnetic radiation emitted by the semiconductor device.
In this case, the radiation behavior of the semiconductor layer can be examined for abrupt, essentially one-dimensional intensity changes along the semiconductor layer, with detected changes of intensity being identified as mechanical defects of the semiconductor device. Such an analysis can be performed automatically by corresponding image processing systems or radiation intensity detectors. Therefore, not only is the global radiation behavior considered here qualitatively (radiation present or not present), but also the intensity course of the radiation behavior is examined locally along the semiconductor device or along the semiconductor layer with the indirect band junction. In this manner, local defects can be detected which impair the operability of the semiconductor device in certain partial ranges or in any other fashion. An abrupt essentially one-dimensional intensity change supports the conclusion that there is a mechanical defect, such as a rupture, of a semiconductor layer.
In principle, the process can also be used to detect local cloudiness, degradations or the like, for example, in cover glass and cover layers of solar cell. However, these may then have an effect, for example, as an intensity change of a two-dimensional nature in the radiation behavior and can be correspondingly detectable.
The present invention achieves a significant improvement of the detection of mechanical defects in planar semiconductor devices, which consist particularly of a semiconductor material with an indirect band junction, such as silicon. In contrast to visual cell inspection, which encounters its limits particularly in the case of texturized silicon solar cells, the detection rate for defects is almost 100% (in contrast to a visual detection rate of, as a rule, less than 50%). Furthermore, the inspection is not dependent on personnel, can be reproduced and permits an unambiguous defect documentation. In addition, the process saves time and can be automated.
According to an embodiment of the invention, a process is provided for detecting mechanical defects of a semiconductor device (particularly, a solar cell or solar cell arrangement) having at least one pn-junction in a semiconductor material characterized by an indirect band junction. For this purpose:                a voltage is applied to the at least one pn junction to operate it in the transmitting direction;        the radiation behavior of the semiconductor layer generated by the applied voltage, at least for partial ranges of the semiconductor layer, can be selected by means of a filter such, that either only the electromagnetic radiation of one or more pn junctions is detected, and/or that all visible ambient light is blocked;        the filtered radiation behavior or, as required, also the unfiltered radiation behavior, is optically detected and analyzed;        the radiation behavior of the semiconductor layer is automatically examined for abrupt, essentially one-dimensional intensity changes along the semiconductor layer, and detected intensity changes are identified as mechanical defects of the semiconductor device; and/or        the integral is automatically determined the radiation intensity of at least a first partial range of a first semiconductor layer, and is automatically compared with an integral determined based on the radiation intensity of at least one comparison range of the same semiconductor layer or of a second semiconductor layer. Based on the result of an automatic comparison the first partial range is identified as defective or as free of defects.        
According to another embodiment of the present invention, direct light incident on a surface of the semiconductor device can be filtered during the optical detection. This has the advantage of detecting and analyzing only that electromagnetic radiation which is emitted by the semiconductor device to which the conducting-state voltage is applied. Electromagnetic radiation which is, for example, reflected by the component and which is generated by light laterally incident on the surface of the semiconductor device, can thus be avoided.
According to another embodiment of the invention, the optical detection can take place within a time period of not more than two seconds. It was found that such a time period is suitable for taking high-resolution pictures of the Si-based semiconductor device for further analysis, using a conventional Si-CCD camera.
In addition, according to a further embodiment of the invention, a voltage for operating the pn junction in the transmitting direction can be applied which causes a current in a range of once or twice the short-circuit current intensity of the semiconductor device. It has been found that such a time period for taking high-resolution pictures of the Si-based semiconductor device is suitable for a further analysis for the detection, using a conventional Si-CCD camera.
According to an embodiment of the invention, the semiconductor material with the indirect band junction may comprise silicon, particularly polycrystalline silicon.
According to a further embodiment of the present invention, mechanical defects of a radiation-inactive layer of the semiconductor device can be detected automatically by examining the radiation behavior of an adjacent radiation-active semiconductor layer for abrupt, essentially one-dimensionally extending intensity changes along the semiconductor layer. Detected intensity changes are identified as mechanical defects of the radiation-inactive layer of the semiconductor device. By means of such an embodiment of the present invention, particularly in the case of tandem solar cells (or solar cells with two or more active layers), a rupture or a degradation of a non-radiating layer of the semiconductor device can also be detected by means of the method according to the invention, because the radiation behavior of the semiconductor device is influenced also by ruptures or degradations in the non-radiating layer.
In addition, according to a further embodiment of the invention, both mechanical defects and the electric effects thereof can be detected, Detection of mechanical defects is based on an analysis of the radiation behavior of the semiconductor device in a first wavelength range, while detection of electric effects is based on an analysis of the radiation behavior of the semiconductor device in a second wavelength range. Because even inactive layers influence the radiation behavior of the semiconductor device, they can also be examined for mechanical defects based on analysis of the radiation behavior of the semiconductor device in this manner.
According to a further embodiment of the present invention, a plurality of n semiconductor devices can be arranged in a series connection and/or parallel connection on a common semiconductor device carrier, and a voltage can be applied to the totality of the semiconductor devices which, in a series connection, corresponds to n-times the voltage (and, in a parallel connection, corresponds to n-times the current) for operation of a semiconductor device in the transmitting direction. As a result, particularly already preassembled solar cell panels or modules having several individual series-connected cells can be inspected.
According to a further embodiment of the invention, an arrangement for the detection of mechanical defects of a semiconductor device (particularly a solar cell or solar cell arrangement) may be provided with at least one pn junction in a semiconductor material characterized by an indirect band junction and at least one electrode connected to each of the respective p-conducting and n-conducting ranges of the semiconductor device, and further including,                voltage supply connections to electrodes of the semiconductor device in order to permit a voltage supply for the operation of the pn junction in the transmitting direction;        an arrangement for optical detection and analysis of the radiation behavior of the semiconductor layer generated by an applied voltage at least for partial ranges of the semiconductor, the arrangement comprising the following characteristics:        an optional filter unit which can be selected only for detecting the radiation of one or more pn junctions and/or for blocking visible ambient light;        an image data storage area for storing a radiation image of a semiconductor layer;        a first program storage area containing an image processing program, constructed for the automatic examination of at least one of the radiation images with respect to abrupt, essentially one-dimensional extending intensity changes and for the identification of the detected intensity changes as mechanical defects of the semiconductor device; and        a second program storage area containing a program for the analysis of at least one of the radiation images with respect to electric effects of the mechanical defects.        
In particular, the optical detection arrangement may also comprise an arrangement for filtering direct light incident on the surface of the semiconductor device, because the detection of the radiation emitted by the semiconductor device may be negatively influenced by reflections of light laterally incident on a surface of the semiconductor device.
In a further embodiment of the present invention, the optical detection arrangement may also be constructed to carry out the optical detection within a time period of not more than two seconds.
According to a further embodiment of the invention, the voltage supply may be constructed to apply an operating voltage to the pn junction in the transmitting direction via the voltage supply connections, which causes a current in a range of approximately once or twice the short-circuit current intensity of the semiconductor device.
Additional advantages and application possibilities of the present invention are contained in the following description in connection with the embodiments illustrated in the drawings.